1. Field of the invention
This invention relates generally to the analysis and process of random pulse signals, issued from a radiation detector, such as a scintillation detector/photomultiplier.
2. The prior art
In a wide variety of communication, telemetering and data processing systems, information from a signal source is transmitted in the form of pulses to detection apparatus which records, counts the number of, or otherwise processes, the transmitted pulses. By way of example, the well logging techniques use such systems. Of the many well logging techniques developed over the years to determine characteristics of the earth formations, such as the hydrocarbon content and productivity, the nuclear spectroscopy tool, by which energy spectra of the constituents of formation matrices and fluids are generated, has been found to provide information of particular value in earth formation analysis. Typically, the energy spectra are obtained by detecting the gamma rays resulting, either from the natural radioactivity, or from interactions of said formations nuclei with high energy neutrons irradiating said formation, and by converting each detected gamma ray into an electrical pulse whose amplitude is a measure of the gamma ray energy. These pulses are then sorted according &.o height in a pulse height analyzer to develop energy spectra characteristic of the earth constituents near the tool.
Generally, the detection of a series of distinct pulses, at low count rate, offers very few problems, and conventional detection equipment may be employed. Often, however, the frequency, or repetition rate, of the pulses varies over a wide range such that the spacing between successive pulses are sometimes very short. Thus, the random character and high rates of occurrence of these signals necessarily produce a "pile-up", or a sequence of overlapping pulses at the amplifier unit. Usually, a pile-up of this sort results in a single pulse that is composed of two or more amplified individual detected signals, each of which is indicative of a detected gamma ray, neutron or other nuclear radiation. The pile-up phenomenon results in data losses and/or spectrum distortion. Accordingly, it is of importance, first to distinguish individual pulses from pile-up pulses, and second, when pile-up pulses have been detected, to adequately process these in order to restore the original distinct pulses, or at least to bring adequate correction to the actual detected signal.
Typical nuclear spectrum analyzers, such as described in the book "Radiation Detection and Measurement" by Glenn F. Knoll (1979), includes, successively, a scintillation detector, a photomultiplier, a coupler (usually a capacitor), a preamplifier, a pulse shaping unit, and a pulse height analyzer; known pulse height analyzers comprise successively a pulse detector (optionally a pile-up detector, and a pile-up process unit), an analog-to-digital converter (ADC) and a memory, the different channels of which correspond to a given amplitude level of the detected pulse; the pulse height analyzer may also comprise, upstream of the ADC, an input gate preventing pulses from reaching the ADC when the latter is busy, i.e. when the ADC is processing a detected pulse. In addition, spectrum analyzers usually comprise a multichannel count device, which determines the time spectra with respect to a reference time, by recording the accumulated counts in each of the time intervals (of e.g. 1 microsecond) of a given measurement time cycle, of e.g. 90 microsecond; such count is usually repeated over several measurement time cycles.
The minimum time interval separating two successive events, which is needed for identifying said events as distinct events, is set by the detector itself, as well as by the different electronic components of the nuclear spectroscopy apparatus. This minimum time interval is usually called "dead time".
A correction method for the losses resulting from the dead time has already been proposed, and which is described in the book by Knoll already referred to, especially pages 95-102. Briefly stated, the correction is based on a mathematical relationship between the true or real count rate "n", the recorded count rate "m", and the dead time "T". One of two mathematical relationships is used depending upon the theoretical model of dead time behavior chosen to represent the actual dead time behavior of the nuclear spectroscopy apparatus in use. Usually, dead time is not accurately known and is determined by empirical measurements, either by observing the counting rate from two sources, individually and in combination, or by using a short-lived radioisotope source, as set forth in the book by Knoll.
These known determination methods can be complex to implement, require specialized and costly equipment, and thus can only be carried out in a laboratory. In addition, most of these methods require stopping the data acquisition pending, to perform the dead time measurement.
Moreover, and above all, the dead time is determined, according to these known methods, under laboratory condition, for each given apparatus, while the dead time actually varies with operating conditions, such as temperature, and also varies from one apparatus to another. By way of example, dead time may vary as much as 50% with temperature, and to an extent of 10% from one apparatus to the other. Some attempts have been made to limit the temperature effects by providing a temperature stabilization device, such as the one described in U.S. Pat. No. 4,620,421 issued to W. K. Brown; this device, nevertheless, increases the complexity of the whole spectroscopy apparatus, without totally obviating temperature effects.
Accordingly, a reliable dead time determination is required, all the more since dead time changes have a substantial effect, not only on the count rates, but also on the measured energy spectrum. However, none of the aforementioned known determination and/or correction methods of the dead time are satisfactory, especially for logging techniques.